From 02ca01e2b7803b666b0647167d9b1ed4fe664ba2 Mon Sep 17 00:00:00 2001
From: brekk <brekk@brekkbockrath.com>
Date: Tue, 2 Apr 2024 14:38:23 -1000
Subject: [PATCH] feat: add `difference` and `symmetricDifference` to List

---
 prelude/__internal__/List.mad      | 24 +++++++++++++++++++++---
 prelude/__internal__/List.spec.mad | 14 ++++++++++++++
 2 files changed, 35 insertions(+), 3 deletions(-)

diff --git a/prelude/__internal__/List.mad b/prelude/__internal__/List.mad
index 94caf449..b60c8728 100644
--- a/prelude/__internal__/List.mad
+++ b/prelude/__internal__/List.mad
@@ -790,9 +790,6 @@ export includes = (x, list) => where(list) {
   [a, ...xs] =>
     a == x || includes(x, xs)
 }
-// a == x ? true : includes(x, xs)
-
-
 
 /**
  * Drops n items from the given list.
@@ -1087,3 +1084,24 @@ export cut = (a, xs) => {
     Nothing
   }
 }
+/**
+ * Finds the set (i.e. no duplicates) of all elements in the first list not contained in the second list. 
+ * @example
+ * difference([1,2,3,4], [7,6,5,4,3]) // [1,2]
+ * difference([7,6,5,4,3], [1,2,3,4]) // [7,6,5]
+ */
+difference :: Eq a => List a -> List a -> List a
+export difference = (z, a) => reduce((stack, c) => includes(c, a) ? stack : [...stack, c], [], z)
+
+/**
+ * Finds the set (i.e. no duplicates) of all elements contained in the first or second list, but not both.
+ * @example
+ * symmetricDifference([1,2,3,4], [7,6,5,4,3]) // [1,2,7,6,5]
+ * symmetricDifference([7,6,5,4,3], [1,2,3,4]) // [7,6,5,1,2]
+ */
+symmetricDifference :: Eq a => List a -> List a -> List a
+export symmetricDifference = (z, a) => reduce(
+  (stack, c) => includes(c, a) && includes(c, z) ? stack : [...stack, c],
+  [],
+  concat(z, a),
+)
diff --git a/prelude/__internal__/List.spec.mad b/prelude/__internal__/List.spec.mad
index 1e330d80..08becd74 100644
--- a/prelude/__internal__/List.spec.mad
+++ b/prelude/__internal__/List.spec.mad
@@ -10,6 +10,7 @@ import {
   concat,
   contains,
   cut,
+  difference,
   drop,
   dropLast,
   dropWhile,
@@ -38,6 +39,7 @@ import {
   sort,
   sortDesc,
   startsWith,
+  symmetricDifference,
   tail,
   tails,
   take,
@@ -291,3 +293,15 @@ test("all - false", () => assertEquals(all((x) => x > 4, [1, 2, 10]), false))
 test("cut", () => assertEquals(cut(5, range(1, 11)), Just(#[[1, 2, 3, 4, 5], [6, 7, 8, 9, 10]])))
 test("cut - negative index", () => assertEquals(cut(-1, range(1, 11)), Nothing))
 test("cut - invalid index", () => assertEquals(cut(16, range(1, 11)), Nothing))
+
+test("difference", () => assertEquals(difference([1, 2, 3, 4], [7, 6, 5, 4, 3]), [1, 2]))
+test("difference - swap", () => assertEquals(difference([7, 6, 5, 4, 3], [1, 2, 3, 4]), [7, 6, 5]))
+
+test(
+  "symmetricDifference",
+  () => assertEquals(symmetricDifference([1, 2, 3, 4], [7, 6, 5, 4, 3]), [1, 2, 7, 6, 5]),
+)
+test(
+  "symmetricDifference",
+  () => assertEquals(symmetricDifference([7, 6, 5, 4, 3], [1, 2, 3, 4]), [7, 6, 5, 1, 2]),
+)